7.2.4.2 Radiative processes in the stratosphere

The stratosphere lies immediately above the troposphere, with the height of
the bounding tropopause varying from about 15 km in the tropics to about 7 km
at high latitudes. The mass of the stratosphere represents only about 10 to
20% of the total atmospheric mass, but changes in stratospheric climate are
important because of their effect on stratospheric chemistry, and because they
enter into the climate change detection problem (Randel and Wu, 1999; Shine
and Forster, 1999). In addition there is a growing realisation that stratospheric
effects can have a detectable and perhaps significant influence on tropospheric
climate.

Solar radiative heating of the stratosphere is mainly from absorption of ultraviolet
(UV) and visible radiation by ozone, along with contributions due to the near-infrared
absorption by carbon dioxide and water vapour. Depletion of the direct and diffuse
solar beams arises from scattering by molecules, aerosols, clouds and surface
(Lacis and Hansen, 1974).

The long-wave process consists of absorption and emission of infrared radiation,
principally by carbon dioxide, methane, nitrous oxide, ozone, water vapour and
halocarbons (CFCs, HFCs, HCFCs, PFCs etc.). The time-scales for the radiative
adjustment of stratospheric temperatures is less than about 50 to 100 days.

For CO2, part of the main 15 micron band is saturated over quite short vertical
distances, so that some of the upwelling radiation reaching the lower stratosphere
originates from the cold upper troposphere. When the CO2 concentration is increased,
the increase in absorbed radiation is quite small and increased emission leads
to a cooling at all heights in the stratosphere. But for gases such as the CFCs,
whose absorption bands are generally in the 8 to 13 micron "atmospheric
window", much of the upwelling radiation originates from the warm lower
troposphere, and a warming of the lower stratosphere results, although there
are exceptions (see Pinnock et al., 1995). Methane and nitrous oxide are in
between. In the upper stratosphere, increases in all well-mixed gases lead to
a cooling as the increased emission becomes greater than the increased absorption.
Equivalent CO2 is the amount of CO2 used in a model calculation that results
in the same radiative forcing of the surface-troposphere system as a mixture
of greenhouse gases (see e.g., IPCC, 1996) but does not work well for stratospheric
temperature changes (Wang et al., 1991; Shine, 1993; WMO, 1999).

An ozone loss leads to a reduction in the solar heating, while the major long-wave
radiative effects from the 9.6 and 14 micron bands (Shine et al., 1995) produce
a cooling tendency in the lower stratosphere and a positive radiative change
above (Ramaswamy et al., 1996; Forster et al., 1997). Large transient loadings
of aerosols in the stratosphere follow volcanic eruptions (IPCC, 1996) which
leads to an increase of the heating in the long-wave. For the solar beam, aerosols
enhance the planetary albedo while the interactions in the near-infrared spectrum
yield a heating which is about one third of the total solar plus long-wave heating
(IPCC, 1996; WMO, 1999). In addition, ozone losses can result from heterogeneous
chemistry occurring on or within sulphate aerosols, and those changes produce
a radiative cooling (Solomon et al., 1996).

The Antarctic ozone hole is a stratospheric phenomenon with a documented impact
on temperature and, during the period 1979 to 1994, ozone decreases very likely
contributed a negative radiative forcing of the troposphere-surface that offset
perhaps as much as one half of the positive radiative forcing attributable to
the increases in CO2 and other greenhouse gases (Hansen et al., 1997; Shine
and Forster, 1999). It appears that most of the observed decreases in upper-tropospheric
and lower-stratospheric temperatures were due to ozone decreases rather than
increased CO2 (Ramaswamy et al., 1996; Tett et al., 1996; Bengtsson et al.,
1999).

The subject of solar effects on climate and weather (see Section
6.10) has enjoyed a recent resurgence, in part because of observational
studies (Labitzke and van Loon, 1997), but more so because of modelling studies
that suggest viable mechanisms involving the stratosphere. As solar irradiance
changes, proportionally much greater changes are found in the ultraviolet which
leads to photochemically induced ozone changes, and the altered UV radiation
changes the stratospheric heating rates per amount of ozone present (Haigh,
1996; Shindell et al., 1999a). Including the altered ozone concentrations gave
an enhanced tropospheric response provided the stratosphere was adequately resolved.